FC Study Guide 2026
Everything you need to pass the FC exam in one place: the exam format, every topic to study, real practice questions with explanations, flashcards, and full-length practice tests. Free, no sign-up needed.
📋 FC Exam Format at a Glance
📚 FC Topics to Study (38)
✍️ Sample FC Questions & Answers
1. When installing a furnace in a closet, which code requirement applies to the combustion air supply?
NFPA 54 requires two permanent openings in confined spaces — one within 12 inches of the top and one within 12 inches of the bottom — to supply adequate combustion air.
2. During a gas furnace installation, the technician discovers the supply gas pressure is 0.25 in. W.C. at the manifold under full load. What action is required?
Natural gas manifold pressure should typically be 3.5 in. W.C.; 0.25 in. W.C. indicates severely insufficient supply pressure requiring the gas utility to investigate the supply line.
3. A furnace cycles on and off frequently (short-cycles) without reaching the set temperature. What is the MOST likely cause?
A clogged air filter restricts airflow over the heat exchanger, causing it to overheat and trip the high-limit switch repeatedly.
4. What is the most common cause of premature heat exchanger failure in residential furnaces?
Chronically restricted airflow (dirty filters, closed registers) causes the heat exchanger to overheat repeatedly, leading to metal fatigue and cracking.
5. What minimum flame signal (microamps DC) is typically required for a furnace flame sensor to keep the gas valve open?
Most furnace control boards require at least 0.5–1.0 µA of DC flame signal to confirm ignition and hold the gas valve open.
6. Which terminal on a conventional thermostat wiring system carries the 24V common (return) signal?
The C (common) terminal completes the 24V circuit from the transformer secondary, providing a return path for connected devices.